1. Field of the Invention
The present invention relates to a WDM (wavelength division multiplexing) system, and in particular to an optical amplifier and a method thereof.
2. Background of the Related Art
Because the need for bandwidth has increased tremendously, it seems impossible to satisfy future bandwidth need with present Internet or asynchronous transfer mode (ATM) technologies. Accordingly, new technologies capable of providing broader bandwidth are required. Optical communication technology has emerged as one type of technology for satisfying these bandwidth needs.
A wavelength division multiple (WDM) method transmits a plurality of channels simultaneously by using different wavelength beams for optical communication.
A wavelength division multiplexing (WDM) optical communication network includes a transmitter for converting data into an optical signal and transmitting it; an optical transmission channel for transmitting the optical signal to a reception side; and a receiver for restoring the optical signal into the original data. The data can be an audio signal, a video signal or digital data.
When long distance communication of an optical signal is performed in an optical communication network, the optical signal is gradually weakened by, for example, noise around an optical transmission channel.
Accordingly, in order to compensate for transmission loss of the optical signal transmitted through the optical transmission channel, an optical amplifier is installed in the optical transmission channel. Among optical amplifiers, a rate-earth element doped optical amplifier using a rare-earth element doped optical fiber as an optical waveguide to amplify light is commonly used.
In a WDM system, the C-band and the L-band are the lowest loss wavelength band of the optical fiber. C-band transmission utilizes the upper and lower 15 nm bandwidth on the basis of 1550 nm band, and L-band transmission utilizes the upper and lower 15 nm bandwidth on the basis of 1590 nm band. Among rare-earth element doped optical fiber amplifiers, an erbium-doped fiber amplifier (EDFA) can amplify light in the C-band and the L-band, which are the lowest loss wavelength bands.
FIG. 1 illustrates a construction of a conventional optical amplifier in a WDM optical communication network. As depicted in FIG. 1, the conventional optical amplifier includes an EDFA 10 for amplifying an input optical signal, a gain flattening filter 20 for flattening a gain of the input optical signal that is amplified in the EDFA 10, a pump laser diode (PLD) 30 for outputting a pump optical signal that provides optical energy for amplifying the optical signal outputted from the gain flattening filter 20, a coupler 40 for combining the optical signal outputted from the gain flattening filter 20 with the pump optical signal outputted from the PLD 30, an isolator 50 for receiving an output optical signal from the coupler 40, outputting it in one direction, removing a portion of the optical signal cut off during the gain flattening, and removing noise that arises during the optical amplification (for example, amplified spontaneous emission (ASE)).
The EDFA 10 includes an isolator 11 for transmitting an input optical signal in one direction, a PLD 12 for outputting a pump optical signal that provides the optical energy required to amplify the input optical signal outputted from the isolator 11, a coupler 13 for combining the input optical signal outputted from the isolator 11 with the pump optical signal outputted from the pump laser diode 12 by a WDM method, and an erbium-doped fiber (EDF) for amplifying the input optical signal using the pump optical signal.
The operation of the conventional optical amplifier will now be described. When the input optical signal is applied, the isolator 11 of the EDFA 10 transmits the input optical signal in one direction to the coupler 13. The PLD 12 outputs the pump optical signal for providing the optical energy required for amplifying the input optical signal.
The coupler 13 combines the input optical signal passing through the isolator 11 with the pump optical signal by a WDM method. The EDF 14 amplifies the input optical signal by using the pump optical signal passing through the coupler 13.
The EDF 14 exhibits gain characteristics that vary as a function of wavelength. Specifically, as depicted in FIG. 2, the amplification gain characteristics of the EDFA 10, are such that the short wavelength band (1520 nm–1570 nm) experiences relatively high gain, and the long wavelength band (1570 nm–1620 nm) experiences relatively low gain.
In a WDM system that includes the EDFA 10, when the wavelength of the optical signal is varied, the optical amplification, the SNR (signal to noise ratio), and thus the transmission quality is varied. As a result, the receiving side of the WDM system experiences difficulties in sensitivity adjustment, noise procession and level adjustment, resulting in a lowering of the performance of the entire system.
In order to solve the above-mentioned problems, the gain flattening filter 20 is used to flatten the gain provided by the EDFA 10. In more detail, the gain flattening filter 20 reduces the amplification (gain) of all the output signals amplified in the EDFA 10 to match the amplification level of the lowest output signal. The gain flattening filter 20 accomplishes this by cutting off any portion of an output signal that is higher than the level of the lowest output signal.
The isolator 50 outputs the output optical signal filtered in the gain flattening filter 20, and absorbs/removes that portion of an optical signal cut off in the gain flattening, as well as spontaneous emission light generated by the optical amplification provided by the EDFA 10.
As described above, based on the wavelength that exhibits the lowest amplification level, the conventional optical amplifier removes optical signal levels that are greater than the lowest amplification level. As a result, the amplification efficiency of the amplifier is lowered.
In addition, a WDM system using both a C-band optical signal and a L-band optical signal includes both a C-band optical signal amplifier and a L-band optical signal amplifier. Because the C-band optical signal amplifier and the L-band optical amplifier respectively include optical components for providing a pump optical signal, it is difficult to reduce the number of the optical components in such a conventional WDM system.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.